The invention relates to transcritical vapor compression devices, one of which is the subject of European patent application No. 89910211.5.
Standard subcritical vapor compression technology requires an operating pressure and temperature well below the critical values of a particular refrigerant. Transcritical vapor compression cycles exceed the critical pressure in the high side of the flow circuit. Since the most important object of the invention is to provide an apparatus and a method facilitating the use of alternatives to environmentally unacceptable refrigerants, the background for the invention is best explained in view of developments from standard vapor compression technology.
The basic components of a single-stage vapor compression system consist of a compressor, a condenser, a throttling or expansion valve, and an evaporator. These basic components may be supplemented with a liquid-to-suction heat exchanger.
The basic subcritical cycle operates as follows. A liquid refrigerant partly vaporizes and cools as its pressure is reduced in the throttling valve. Entering the evaporator, the mixed liquid-vapor refrigerant absorbs heat from a fluid being cooled and the refrigerant boils and completely vaporizes. The low-pressure vapor is then drawn into a compressor, where the pressure is raised to a point where the superheated vapor can be condensed by the available cooling media. The compressed vapor then flows into the condenser, where the vapor cools and liquefies as the heat is transferred to air, water or another cooling fluid. The liquid then flows to the throttling valve.
The term "transcritical cycle" denotes a refrigeration cycle operating partly below and partly above the refrigerant's critical pressure. In the supercritical region, pressure is more or less independent of temperature since there is no longer any saturation condition. Pressure can therefore be freely chosen as a design variable. Downstream from the compressor outlet, the refrigerant is cooled at mainly constant pressure by heat exchange with a coolant. The cooling gradually increases the density of the single phase refrigerant.
A change in volume and/or instant refrigerant charge in the high side affects the pressure, which is determined by the relation between the instant charge and the volume.
In contrast, subcritical systems operate below the refrigerant's critical point and therefore operate with two phase conditions in the condenser, saturated liquid and vapor. A change in the volume of the high side will not directly affect the equilibrium saturation pressure.
In transcritical cycles the high side pressure can be modulated to control capacity or to optimize the coefficient of performance, and the modulation is done by regulating the refrigerant charge and/or regulating the total internal high side volume of the system.
WO-A-90/07683 discloses one of these options for control of the supercritical high side pressure, namely variation of the instant refrigerant charge in the high side of the circuit.
From DE-C-898 751 it is known to apply a high pressure liquid accumulator in order to maintain the refrigerating capacity and to even out the low side temperature fluctuations during the compressor off periods. The disclosure is related to the system operating at subcritical high side pressure having different purpose and mechanism compared to the present control of the supercritical high side pressure.